In aircraft breathing systems where oxygen enrichment may be achieved through on-board oxygen generation, one means of generating an oxygen rich product gas is by fractionalizing air. Air fractionalizing is usually accomplished by alternataing the flow of high pressure air through each of two beds of molecular sieve material such as a zeolite. This process is identified as the pressure swing adsorption technique.
The oxygen concentration of the enriched product gas of a pressure swing adsorption system decreases as the product gas flow through the system increases. It is therefore possible to control the oxygen partial pressure (PPO.sub.2) within a fixed range or vary the PPO.sub.2 in accordance with other parameters such as breathing system pressure.